Layout data lost on +error.svelte

[ReindDooyeweerd]

Describe the bug

I noticed something yesterday after upgrading on of my SvelteKit projects from 2.37.0 to 2.37.1.

I created a repo that reproduces the issue as described below, repo can be found here: https://github.com/ReindDooyeweerd/sveltekitbug

The issue:
In my +layout.server.ts i normally fetch some data that is used throughout the whole website like menu data etc, this works great.

In the repo posted above i created a [slug] with a +page.server.ts which also fetches some data so https://localhost:5173/ shows the data fetched in the layout and https://localhost:5173/a shows both the data from +layout.server.ts and +page.server.ts => https://share.cleanshot.com/FV2B0qQYx2nNRLFS7mfQ

So far so good. Then i created a [slug_2] inside [slug], added a fetch also BUT i make sure that doesn't return any data and throw a 404. So https://localhost:5173/a/b should end up on +error.svelte

if (!fetchedPageData || Object.keys(fetchedPageData).length === 0) {
  error(404, 'Page not found');
}

This renders the +error.svelte but now the weird stuff happens since 2.37.1, the layout data becomes empty also: https://share.cleanshot.com/fx5VwqlRWJ67cLlKMKmz

While on version 2.37.0 with the exact same code i get this: https://share.cleanshot.com/x5bFFgz84gTjNJxVxjpR

Reproduction

1. Clone https://github.com/ReindDooyeweerd/sveltekitbug
2. npm install && npm run dev
3. visit https://localhost:5173 => layout data in pre tags
4. visit https://localhost:5173/a => layout data + page data in pre tags
5. visit https://localhost:5173/a/b => fetch is empty, +error.svelte is rendered and both pre tags are empty
6. change sveltekit version to 2.37.0 in package.json and start from 2 and on 5. you will see that the layout data is still available while pagedata is empty as expected

Version of the repo on stackblitz: https://stackblitz.com/~/github.com/ReindDooyeweerd/sveltekitbug

Logs

Nothing in the browser console or terminal

System Info

System:
    OS: macOS 15.6.1
    CPU: (10) arm64 Apple M2 Pro
    Memory: 641.34 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.17.0 - ~/.nvm/versions/node/v22.17.0/bin/node
    npm: 10.9.2 - ~/.nvm/versions/node/v22.17.0/bin/npm
  Browsers:
    Chrome: 140.0.7339.80
    Safari: 18.6
  npmPackages:
    @sveltejs/adapter-auto: ^6.1.0 => 6.1.0 
    @sveltejs/kit: ^2.37.0 => 2.37.1 
    @sveltejs/vite-plugin-svelte: ^6.1.4 => 6.1.4 
    svelte: ^5.38.7 => 5.38.7 
    vite: ^7.1.5 => 7.1.5

Severity

blocking an upgrade

Additional Information